A combination of lack of demand for development snapshots, and lack of
a volunteer to make them.

Well, raw unstable dev snapshot tarballs are often just automagically
handled by web frontend interfaces to VCS, human intervention not
required, server just tars up an export of a revision from the repo.

Okay, apparently /not/ in the case of bzr on savannah - and n.b. the
functionality is still lacking from launchpad/loggerhead upstream [1]
(oddly), so reenabling the apparently disabled loggerhead on savannah
wouldn't help.

However, circuitously, the web frontend to an unofficial import of emacs
into git does happen to make them [2][3]

*** Of course, _don't_ use such a snapshot unless you're prepared for
development code that might eat your socks or worse.